docs: improve/add documentation of Lua types

- Added `@inlinedoc` so single use Lua types can be inlined into the
  functions docs. E.g.

  ```lua
  --- @class myopts
  --- @inlinedoc
  ---
  --- Documentation for some field
  --- @field somefield integer

  --- @param opts myOpts
  function foo(opts)
  end
  ```

  Will be rendered as

  ```
  foo(opts)

    Parameters:
      - {opts} (table) Object with the fields:
               - somefield (integer) Documentation
                 for some field
  ```

- Marked many classes with with `@nodoc` or `(private)`.
  We can eventually introduce these when we want to.

scripts/gen_help_html.lua

@@ -752,7 +752,7 @@ end
 ---
 --- @param fname string help file to parse
 --- @param parser_path string? path to non-default vimdoc.so
---- @return LanguageTree, integer (lang_tree, bufnr)
+--- @return vim.treesitter.LanguageTree, integer (lang_tree, bufnr)
 local function parse_buf(fname, parser_path)
   local buf ---@type integer
   if type(fname) == 'string' then